Recordings of motion video of a scene at different qualities, which often are initiated by external triggers, by events in the scene, by environmental changes, scheduled recordings, etc., has become more and more common in surveillance and monitoring systems including motion video cameras. One reason for the differentiated video quality is to save storage space, which may be a rare commodity in the systems, at least in view of motion video recordings which are known to require a lot of storage space. In systems having a plurality of users the differentiated video quality may be a result of different requirements and different main interests between different users or different display equipment.
For example, a video camera monitoring a shop may be arranged to constantly record a low resolution motion video, to record a higher resolution motion video when movement is detected in specified parts of the scene, and to record an even higher resolution motion video when an alarm is triggered by an external sensor. Hence, a plurality of motion video recordings of the same scene during overlapping time periods is stored.
An operator of such a surveillance or monitoring system is occasionally interested in playing back recorded motion video starting at a specific point in time. The operator then has to select the video recording that is most likely to include video from a time period including this point in time. If the operator want to study, for example, a period starting some time before an motion detection or an alarm, the operator would have to display the low resolution motion video and if then when the playback reaches a point in time when a higher resolution motion video exists the operator has to manually switch to the higher resolution motion video or keep watching the low resolution motion video stream.
In UK patent application GB 2 427 521 there is provided an image display method including an image reproduction function capable of effectively reproducing image data recoded in various forms in a storage device. The application describes a system where consecutive pictures from one camera are stored on an image storage and delivery server. Two channels are implemented in the image storage and delivery server, each for images from the same camera but stored at different rates, e.g. a normal channel (low frame rate) and an alarm channel (high frame rate). A method described in the application describes reproducing images on a monitor of a client device according to a method called “conducting seamless reproduction” taking into account images from each of the channels. This method operates by reproducing images from the alarm channel during the time periods when there are images in the alarm channel and then for the time period when there are no longer any images in the alarm channel but when there are images in the normal channel, the method reproduces the images from the normal channel. For time periods when there are no images in any of the channels it is possible to skip reproducing until image data is present in any of the channels again. The reason why reproduction of the image data in the alarm channel is provided with priority is that it is possible to ascertain contents in more detail by reproducing the image data in the alarm channel having the image data recorded at high frame rate. The described system have the client device check time stamps of images in the image storage and delivery server and then request images with a calculated frame number, which may be from both the normal channel and the alarm channel, then the client selects the image to be reproduced.
One of the problems with a method and system as described above is that it is quite complex, memory demanding and processing intensive.